Package-on-Package (PoP) structures are integrated circuit (IC) package structures where two or more IC packages are stacked or placed on top of one another. Usually, these stacked IC packages would have interconnections that connect and route signals between them. Typical PoP configurations include two or more memory packages stacked on top of one another or a memory package stacked on top of a logic package. For instance, a logic package with a higher I/O (input/output) density may be placed at the bottom of the PoP structure, while a memory package with a lower I/O density may be placed on top of the logic package. Such a configuration is commonly used in a cell phone where the logic package could be an application processor for the cell phone.
However, conventional PoP structures may have a low assembly yield due to the low stacking yield during the stacking process. One of the common causes of the low yield in PoP assembly is ‘solder ball shear’. Solder balls placed on the perimeter of the top surface of a package substrate are generally used to connect two or more packages together. However, when one IC package is mounted on another IC package, the solder balls may break or shear. This is mainly caused by warpage on the bottom IC package because the bond between the solder balls and the bottom IC package is not strong enough to hold the weight of the top IC package.
Apart from the low assembly yield, because the solder balls that connect the top and bottom packages are only placed on the perimeter area of the bottom package substrate, common ball grid array (BGA) packages cannot be mounted on top of a conventional PoP structure. Therefore, the top IC package may require a special ball layout in order to be mounted on the bottom IC package. This greatly limits the ball layout on the top package and also the number of packages that can be mounted.
As such, it is desirable to have a PoP structure that would not limit the types of IC packages that can be placed on top. It is also desirable to have a stronger and more stable structure that can accommodate the weight of one or more IC packages on top of another. It is within this context that the invention arises.